Fiery

Fiery trait makes mob immune to fire, ignite attackers, and ignite targets attacked.

Mob will have traits when they have high enough level. At first, player may only find mobs with rank 1 traits. When player kills a mob with rank 1 trait, rank 2 traits will be unlocked for mobs spawned near this player.

Traits have their own loot table. You can check JEI for drops of each trait.

The number of traits a mob can have depends on level and trait ranks. When the maximum trait rank is low while mob level is high, the mob would have a lot of traits. Conversely, if the mob has low level but is allowed to have high rank traits, the mob may have only one trait.